The selective C—H bond activation and catalytic functionalization of organic molecules is a simple and environmentally benign route for the production of valuable small molecules as well as for the late-stage functionalization of complex chemical architectures.[1-3] Among the different metal-catalyzed C—H functionalization systems, the borylation of organic compounds is a highly important reaction that gives access to valuable chemicals that can be used in opto-electronic systems, in pharmaceuticals or as reagents in processes such as the Suzuki-Miyaura cross-coupling and the Chan-Lam reaction.[4-9]
In recent years, iridium/bipyridine catalytic systems have surpassed other noble metal catalysts as the most reliable and convenient mediators[5, 6, 10-13] for selective C—H bond activation, although these catalysts generate undesirable costs and potential risks to humans.[14] Base-metal alternatives, such as iron, iron/copper, cobalt and nickel catalysts have been reported, but suffer from inferior efficiency as compared to precious metal systems.[15-19]
Electrophilic borylation strategies have also recently emerged but in the current state of things, the generation of boron cations necessitates the use of stoichiometric reagents[20-22] or transition metal-based catalysts[23] or strongly acidic[24] catalysts.
A concept that has attracted considerable attention of late is that of frustrated Lewis pairs (FLPs) as metal-free catalytic systems.[25-28] Indeed, this strategy has been shown to achieve a wide range of reactivity previously exclusive to transition metals in the fields of hydrogenation,[29-37] hydroboration[38-44] and hydrosilylation[45, 46].
More recently, the scope of the FLP reactions has been expanded out of the traditional field of reduction processes and the use of ambiphilic 1-TMP-2-borylbenzene (TMP=2,2,6,6-tetramethylpiperidine)[47] as a metal-free catalyst for the cleavage and borylation of heteroaromatic C—H bonds was reported.[48] Mechanistic investigations have suggested that this catalyst relied on the interaction of the aromatic substrate with the boron atom of 1-TMP-2-borylbenzene, with simultaneous deprotonation by the basic TMP moiety. This mechanism was likened to the “concerted metalation-deprotonation” (CMD) that had been previously proposed by Fagnou and coworkers as the dominant process in the palladium carboxylate-mediated direct arylation reaction.[49] Interestingly, the selectivity of the reaction was found to be dominated by the nucleophilicity of the aromatic substrate and to be complementary to that of most iridium-based systems where the activation is guided by the acidity of the proton to be cleaved.[12, 50]
While metal-free catalysts for C—H activation are an exciting idea, because of their low cost and low toxicity, the applicability of 1-TMP-2-borylbenzene for borylation reactions remains rather limited in view of the moisture sensitivity of the BH2 moiety. The moisture sensitivity, a constant in most FLP chemistry, implies a necessity for handling and storing the catalyst under an inert atmosphere and represents an important obstacle to its implementation.[51]
The present disclosure refers to a number of documents, the contents of which are herein incorporated by reference in their entirety.